The industrial revolution and, most recently, new technology involving nuclear fission have introduced new toxicological problems into the environment. Heavy metals and radioactive isotopes of elements are today being handled in unprecedented quantities and concentrations. The metal toxicity and radioactivity associated with long-lived high-radiation isotopes are particularly of great concern. Indeed, the disposal problems presented by radioactive isotopes originating directly or indirectly from the operation of nuclear facilities have produced a strong resistance to this technology.
Similarly, years of metals contamination from improperly designed facilities, accidental spills and intentional dumping have resulted in substantial inventories of undesirable metallic species in soil, often covering large areas. This can lead to accumulation of trace elements, especially toxic heavy metals, in the food chain, thereby resulting in unacceptably high concentrations of contaminants in animals and plants. Other types of accidents, such as the nuclear reactor accident at Chernobyl, can cause radioactive fallout which contaminates hundreds of square miles of surrounding land, albeit only to a depth of a few inches of the topsoil.
The ability of such trace metallic elements to spread and further contaminate surrounding areas is governed by basic physicochemical phenomena. For example, soil pH can play an important role in the mobility of some metallic elements, particularly the group comprising cobalt, copper, mercury, nickel, silver, and uranium. The mobility of those elements is related to their solubility in water. The group comprising molybdenum, selenium, uranium, and vanadium mobilizes readily under oxidizing conditions since in all cases the higher oxidation states of these elements are much more mobile.
Previously, electrokinetic soil processing using currents on the order of milliamps per cm.sup.2 of electrode area has been demonstrated for removal/separation of organic and inorganic contaminants and radionuclides. Electrokinetics, which relates to the motion of particles or liquids resulting from a difference of electric potential, as used herein can include electrophoresis, electro-osmosis and electrolytic conduction. Electrophoresis is the movement of suspended particles, e.g., colloidal particles, charged particles, clay particles and such, through a fluid such as a pore liquid under the action of an electromotive force. Electro-osmosis is the movement of a pore fluid due to the influence of an electric field, and electrolytic conduction relates to the movement of simple ions due to the influence of an electric field.
There is limited literature concerning removal of ions from soils by the electrokinetic phenomena. One of the earliest studies is by Jacobs and Mortland (1959), who demonstrated that alkali metals can be extracted from clays by electrokinetics. In 1980, Segal reported that heavy metal concentration substantially increased in an effluent after subjecting same to an electropotential. Hamnet (1980) showed that salts in agricultural soils could be moved by electrokinetics. Shmakin (1985) noted that electrokinetics have been used in the Soviet Union since the early 1970's as a method for concentrating metals and exploring for minerals in deep soil deposits. In 1986, Runnels and Larson demonstrated that CuSO.sub.4 could be extracted with an electropotential gradient. U.S. Pat. No. 4,935,114 to Varma discloses the use of microwave-enhanced chemical processes for disposal of toxic waste including chlorinated hydrocarbons, in which non-metallic particulates having a high dielectric loss factor and toxic wastes are subjected to a temperature in excess of 400.degree. C. in the presence of microwave radiation. U.S. Pat. No. 4,645,004 to Bridges et al. discloses an electro-osmotic method for producing hydrocarbons utilizing in-situ heating of earth formations having substantial electrical conductivity.
Notwithstanding the aforementioned limited successes, the efficiency of electrokinetic extraction procedures has been found to be compromised by ion insolubility or by ion immobilization, most notably when soil clays are involved. In such instances, contaminant ions (generally metal) which are associated with the clay, generally by adsorption in one or more of a multitude of complex states, will be extremely difficult to remove by electrokinetic means.
In certain instances, metals can be mobilized by electropotential only after altering the chemical complexation and/or valence state of the ion by means of chemicals which are applied to the soil. There are, however, serious limitations to altering the behavior of the soil clay/metal ion kinetics via introduction of chemicals. Such limitations can include, inter alia, the massive quantity of chemicals required and the affect of the chemicals on the environment. In most cases, such chemicals participate in competing reactions, which are often unnecessary and even undesirable. Thus, the ultimate effect of treating soil with such chemicals may be directly contrary to the overall objective of environmental remediation.
In other instances, soil can be physically removed from the ground and subjected to "soil washing" in which chemicals are used to try to remove contaminants or other species from the soil.
At other times, it may be desired to remove one or more associated species because of the intrinsic value of the species. For example, it may be desired to obtain certain minerals which are present in soil, and which are adsorbed, entrapped or otherwise associated with the soil.
At still other times, it may be desirable to enhance an already existing chemical, mechanical or electrical process for dissociating species within a matrix.
Accordingly, there is a continuing need for methods for effecting or enhancing the dissociation of species such as contaminants or desirable metals which are associated with a matrix such as soil. This is especially true where, as discussed above, the contaminant is a radioactive isotope and the soil is a soil clay.
Similarly, there is a also a continuing need for methods for effecting or enhancing the dissociation and/or migration of a species associated with a matrix. Such methods would be particularly useful, for example, for decontaminating soil, concentrating and/or confining contaminants within soil, and removing or concentrating desirable minerals within soil.
There is also a continuing need for effecting or enhancing such dissociation and/or migration for species which are confined generally near the surface of the matrix, for example, the topsoil surrounding Chernobyl, as well as those which are found generally at all depths or just at deeper depths.